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Chapter 8 YUM - Notes

8.2 Introduction

yum program provides higher level of intelligent services for using underlying rpm program. Can automatically resolve dependencies when installing, updating, removing packages. Accesses external software repositories, synchronizing with them, retrieving/installing software as needed.

8.3 Learning Objectives:

  • Discuss package installers and their characteristics.
  • Explain how yum works as a high level package management system.
  • Configure yum to use repositories.
  • Discuss the queries yum can be used for.
  • Verify, install, remove, and upgrade packages using yum.
  • Learn about additional commands and how to install new repositories.
  • Understand how to use dnf, which has replaced yum on Fedora.

8.4 Package Installers

Lower-level package utilities (eg. rpm, dpkg) deal with details of installing specific software package files. managing already installed software.

Higher-level package management systems (eg. yum, dnf, apt, zypper) work with databases of available software, incorporate tools needed to find, install, update, uninstall software in highly intelligent fashion.

  • Can use both local/remote repositories as source to install/update binary as well as source software packages
  • Used to automate install, upgrade, removal of software packages
  • Resolve dependencies automatically
  • Save time because no need to either download packages manually/search out dependency information separately

Software repositories provided by distributions/other independent software providers. Package installers maintain databases of available software derived from catalogs kept by repositories. Unlike low-level package tools, have ability to find/install dependencies automatically -> critical feature.

In this section, discuss yum and dnf. zypper and apt discussed in later chapters.

8.5 What is yum?

yum provides frontend to rpm. Primary task: fetch packages from multiple remote repositories, resolve dependencies among packages. Used by majority (but not all) of distributions using rpm, including RHEL, CentOS, Scientific Linux, Fedora.

yum caches information + databases to speed up performance. To remove some or all cached information, can run command:

$ yum clean [ packages | metadata | expire-cache | rpmdb | plugins | all ]

yum has number of modular expressions (plugins) + companion programs that can be found under /usr/bin/yum* and /usr/sbin/yum*.

Will concentrate on command line use of yum, not consider graphical interfaces distributions provide.

8.6 Configuring yum to Use Repositories

Repository configuration files kept in /etc/yum.repos.d, have .repo extension. Eg. on one RHEL 7 system: yumrepos

Note: on RHEL 6 there is no redhat.repo file. RHEL 6 + earlier versions handled distribution-supplied repos in somewhat different manner, although RHEL clones like CentOS used conventional repos for main distribution packages.

8.7 Repository Files

Very simple repository file may look like: 

[repo-name]
    name=Description of the repository
    baseurl=http://somesystem.com/path/to/repo
    enabled=1
 More complicated examples found in `/etc/yum.repos.d`, would be good idea to examine them.

Can toggle the use of particular repository on/off by changing value of enabled to 1/0, or using --disablerepo=somerepo and --enablerepo=somerepo options when using yum.

Can (but should not) also turn off integrity checking with gpgcheck variable.

8.8 Queries

Like rpm, yum can be used for queries such as searches. However, can search not just what is present on local system, but also inquire about remote repositories. Examples:

  • Search for packages with keyword in name:

    $ sudo yum search keyword
$ sudo yum list "*keyword*"

    These two commands give somewhat different information. First one tells more about packages, second one makes it clearer what is installed, what else is available.

  • Display information about a package:

    $ sudo yum info package

    Information includes size, version, what repository it came from, source URL, longer description. Wildcards can be given, eg. yum info "libc*" for this + most yum commands. Note: package need not be installed, unlike queries make with rpm -q.

More yum examples:

  • List all packages, or just those installed, available, or updates that have not yet been installed:

    $ sudo yum list [ installed | updates | available ]

  • Show information about package groups installed or available, etc.:

    $ sudo yum grouplist [group1] [group2]
$ sudo yum groupinfo group1 [group2]

  • Show packages that contain a certain file name:

    $ sudo yum provides

    as in

    $ sudo yum provides "/logrotate.conf"

    Note need to use at least one / in file name, which can be confusing.

8.9 Verifying Packages

Package verification requires installation of yum-plugin-verify package. Might have to do:

$ sudo yum install yum-plugin-verify

Note: this is yum plugin, not executable. Many other plugins available for yum, extends possible set of commands and arguments it can take:

  • To verify package, giving most information:

    $ sudo yum verify [package]

  • To mimic rpm -V exactly:

    $ sudo yum verify-rpm [package]

  • To list all differences, including configuration files:

    $ sudo yum verify-all [package]

Without arguments, above commands will verify all packages installed on system.

By default, verification commands ignore configuration files which may change through normal + safe usage. Some other options: see man yum-verify.

8.10 Installing/Removing/Upgrading Packages

Some examples of commonly performed operations:

  • Install one or more packages from repositories, resolving/installing any necessary dependencies:

    $ sudo yum install package1 [package2]

  • Install from local rpm:

    $ sudo yum localinstall package-file

    This is not quite the same as

    $ rpm -i package-file

    because it will attempt to resolve dependencies by accessing remote repositories.

  • Install specific software group from repository, resolving/installing any necessary dependencies for each package in group:

    $ sudo yum groupinstall group-name

    or

    $ sudo yum install @group-name

  • Remove packages from system:

    $ sudo yum remove package1 [package2]

    Must be careful with package removal, as yum will not only remove requested packages, but all packages that depend on them! May not be what you want, so never run yum remove with -y option, which assumes automatic confirmation of removal.

  • Update package from repository:

    $ sudo yum update [package]

    If not package name given, all packages updated.

During installation (or update), if package has configuration file which is updated, will rename old configuration file with .rpmsave extension. If old configuration file will still work with new software, will name new configuration file with .rpmnew extension. Can search for these filename extensions (almost always in /etc subdirectory tree) to see if you need to do any reconciliation, by doing:

$ sudo find /etc -name "*.rpm*"

Same behavior the more naked underlying rpm utility exhibits, but mentioned here for reference.

8.11 Additional Commands

No shortage of additional capabilities for yum, according to what plugins are installed. Can list them all with:

$ sudo yum list "yum-plugin"

In particular:

  • Show list of all enabled repositories:

    $ sudo yum repolist

  • Initiate interactive shell in which to run multiple YUM commands:

    $ sudo yum shell [text-file]

    If text-file given, yum will read + execute commands from that file instead of from terminal.

More examples of yum commands:

  • Download package, but do not install them; just store them under the /var/cache/yum directory, or another directory specified:

    $ sudo yum install --downloadonly package

    or can type "d" instead of "y" or "n" when prompted after issuing install command. Package(s) will be downloaded under /var/cache/yum in location depending on repository from which download proceeds, unless --downloaddir= option used. Any other necessary packages will also be downloaded to satisfy dependencies.

  • Can view history of yum commands, and, with correct options, even undo/redo previous commands:

    $ sudo yum history

8.12 dnf

dnf intended to be next generation replacement for yum, will underlie yum in RHEL 8.

Can gradually learn to use dnf on Fedora systems because it accepts subset of yum commands that take care of majority of day-to-day tasks + points out at each use of yum that has dnf equivalent.

To learn more, see: Package Management section in the Fedora System Administrator's Guide.

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